It is well known to detect current by measuring the voltage across a low resistivity sense resistor, then calculating current by dividing the voltage by the nominal resistance. This is frequently done in switching power supplies where switches are controlled based on a current through a sense resistor crossing a threshold level to achieve a target output voltage.
Generally, the resistance of all sense resistors is somewhat sensitive to temperature (i.e., the resistor has a temperature coefficient of resistance).
In switching power supplies, an inductor is used in the output filter. The copper winding may be used as the sense resistor. Thus no additional voltage drop is incurred, and no additional real estate is needed for the sense resistor. However, the resistance of the inductor winding is relatively sensitive to temperature fluctuations. Therefore, the sensed current through the winding will not be accurate as the temperature varies from a reference temperature used to set the resistance value of the winding.
It is known to provide a temperature sensor, such as a pn junction device, proximate to the sense resistor in an attempt to determine the actual temperature of the sense resistor and correct for any deviation in resistance due to the actual temperature differing from the reference temperature. The actual temperature of the sense resistor may fluctuate due to the changing power dissipated by the sense resistor. Since the sense resistor itself is generating heat, and the temperature sensor is separated from the sense resistor, the temperature sensor does not accurately sense the actual temperature of the sense resistor. The temperature of the sense resistor may fluctuate due to changes in power, and the medium between the sense resistor and the temperature sensor cannot quickly conduct the resistor temperature to the temperature sensor due to the thermal time constant of the medium. The medium may be a printed circuit board, silicon, a ceramic substrate, etc.
What is needed is a more accurate technique for estimating the actual temperature of the sense resistor so that the actual resistance of the sense resistor can be better estimated when calculating the current through the sense resistor.